How does


cathodic protection stop corrosion?


Cathodic protection prevents corrosion by converting all of the anodic (active) sites on the metal surface to cathodic (passive) sites by supplying electrical current (or free electrons) from an alternate source.


Usually this takes the form of anodes, which are more active than steel. This practice is also referred to as a sacrificial system, since the anodes sacrifice themselves to protect the structural steel from corrosion.


In the case of aluminium anodes, the reaction at the aluminium surface is: (four aluminium ions plus twelve free electrons)


4Al => 4AL+++ + 12 e-


and at the steel surface: (oxygen gas converted to oxygen ions which combine with water to form hydroxyl ions).


3O2 + 12e- + 6H2 O => 12OH-


As long as the current (free electrons) arrives at the cathode (steel) faster than oxygen is arriving, no corrosion will occur.


The electrical current that an anode discharges is controlled by Ohm’s law, which is:


I=E/R I = Current flow in amps


E = Difference in potential between the anode and cathode in volts


R = Total circuit resistance in ohms


Initially, current will be high because the difference in potential between the anode and cathode are high but as the potential difference decreases due to the effect of the current flow onto the cathode, the current gradually decreases due to polarization of the cathode.


The circuit resistance includes both the water path and the metal path, which includes any cable in the circuit. The dominant value here is the resistance of the anode to the seawater.


For most applications, the metal resistance is so small compared to the water resistance that it can be ignored (although that is not true for sleds or long pipelines protected from both ends). In general, long, thin anodes have lower resistance than short, fat anodes.


They will discharge more current but will not last as long.


Therefore, a cathodic-protection system designer must recommend anodes so that they have the right shape and surface area to discharge enough current to protect the structure and enough weight to last the desired lifetime when discharging this current.


As a general rule of thumb:


The weight of the anode determines how much current the anode can produce, and consequently, how many square metres of steel can be protected.


Galvanic corrosion - often misnamed electrolysis - is one common form of corrosion in marine environments.


known as the galvanic series. For information, the galvanic series is given in the table below.


It is a list


of metals and alloys ranked in order of their tendency to corrode in marine environments.


Metals at the top of the scale are called noble and those at the bottom base. The galvanic series can be used to predict which metal will become the anode and how rapidly it will corrode. If any two metals from the list are coupled together, the one closer to the anodic (or active) end of the series, the upper end in this case, will be the anode and thus will corrode faster, while the one toward the cathodic (or noble) end will corrode slower. For example, suppose an aluminium alloy with a voltage range of -0.7 to -0.9 V (an average of -0.8 V) as shown on the series, is coupled to a 300 series stainless steel with an average voltage of -0.07 V.


The galvanic series predicts that aluminium will be the anode, and the voltage difference between the two alloys will be about 0.73 V (obtained by subtracting the two average voltages). It is this voltage difference that drives the current flow to accelerate corrosion of the anodic metal.


It occurs


when two (or more) dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.


Either (or both) metal in the couple may or may not corrode by itself (themselves) in seawater. When contact with a dissimilar metal is made, however, the self corrosion rates will change: corrosion of the anode will accelerate; corrosion of the cathode will decelerate or even stop. The choice of material depends upon the position of the metals concerned in what is


The two major factors affecting the severity of galvanic corrosion are:


• the voltage difference between the two metals on the galvanic series


• the size of the exposed area of cathodic metal relative to that of the anodic metal.


The weight of the anode determines how much current the anode can produce, and consequently, how many square metres of steel can be protected.


The Report • March 2017 • Issue 79 | 37


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